Apparatus and method for utilization of a high-speed optical wavelength tuning source

ABSTRACT

Exemplary embodiments of apparatus, source arrangement and method for, e.g., providing high-speed wavelength tuning can be provided. According to one exemplary embodiment, at least one arrangement can be provided which is configured to emit an electromagnetic radiation that (i) has a spectrum whose mean frequency changes at an absolute rate that is greater than about 6000 (or 2000) terahertz per millisecond, (ii) whose mean frequency changes over a range that is greater than about 10 terahertz, and/or (iii) has an instantaneous line width that is less than about 15 gigahertz. According to another exemplary embodiment, at least one arrangement can be provided configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, with the periodic selection being performed at a first characteristic period. The mean frequency can vary linearly over time, wherein the apparatus can emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period. Further, the first characteristic period can be greater than the second characteristic period.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority from U.S. Patent Application Ser. No. 61/149,922, filed on Feb. 4, 2009, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relates generally to optical systems, and more particularly to an optical wavelength filter system and method for wavelength tuning and a wavelength-swept laser arrangement and methods which can utilize the optical wavelength filter system.

BACKGROUND INFORMATION

Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been provided in an extended-cavity semiconductor laser by using a diffraction grating filter arrangement. To obtain a single-frequency laser operation and to ensure a mode-hop-free tuning, however, a complicated mechanical apparatus and limit the maximum tuning speed may need to be used, conventionally. One of the fastest tuning speeds demonstrated using the conventional systems has been limited to less than 100 nm/s. In certain applications, such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width can provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy.

A line width on the order of about 10 GHz is achievable with the use of an intra-cavity tuning element (such as, e.g., an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously provided has been less than about 1 kHz, limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 15 kHz may be needed for video-rate (e.g., greater than 30 frames/second), high-resolution optical imaging in biomedical applications.

Further, a wavelength-swept laser has been described which can use polygon scanning filter, and that can provide high-speed wavelength tuning up to about 10,000 nm/ms. While the high-speed polygon based wavelength-swept light source facilitates high-speed imaging as fast as about 200 frames/s, wavelength tuning rate as fast as about 10,000 nm/ms, maintaining an instantaneous line-width narrower than 0.15 nm has already reached to the limit of the current polygon based wavelength-swept filter.

Indeed, one of the objects of the exemplary embodiments of the present disclosure is to reduce or address the deficiencies and/or limitations of the prior art procedures and systems described herein above. For example, with respect to faster tuning, wide wavelength tuning range and narrow instantaneous line-width at fast tuning rate, there may be a need for an exemplary embodiment of wavelength scanning filter arrangement and procedure (e.g., a laser procedure).

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relate to apparatus, a source arrangement and method for light-wave filtering that can provide a high-speed wavelength-swept light with a broad spectral tuning range and a narrow instantaneous line-width. In one exemplary embodiment of the present disclosure, the exemplary high-speed wavelength-swept laser can include a high-finesse wavelength tuning filter which may use a polygon scanning mirror, a short length laser resonator with a semiconductor optical amplifier (SOA) gain medium, a time interleaving optical delay line, and/or a booster optical amplifier, e.g., at an output port of the source. Certain exemplary optical components and exemplary arrangement and a short length laser cavity can facilitate a high-speed wavelength sweep over a broad tuning range with a narrow instantaneous line-width. In one exemplary configuration, wavelength variation rates greater than about 41,000 nm/ms with an instantaneous line-width narrower than about 0.2 nm can be obtained. The exemplary laser resonator can include, e.g., a unidirectional ring, or a linear cavity, with a particularly-designed semiconductor optical gain medium to maximize the gain and to minimize the cavity length of the laser cavity.

Thus, an exemplary embodiment of an apparatus according to the present disclosure can be provided. In this exemplary embodiment, at least one arrangement can be provided which is configured to emit an electromagnetic radiation. Such exemplary radiation can have a spectrum whose mean frequency changes (i) at an absolute rate that is greater than about 6000 terahertz per millisecond, and (ii) over a range that is greater than about 10 terahertz.

For example, the mean frequency can change repeatedly at a repetition rate that is greater than 5 kilohertz. The spectrum can have a tuning range whose center is approximately centered at 1300 nm. The spectrum can have an instantaneous line width that is smaller than 100 gigahertz, or even 35 gigahertz. A polygon arrangement can be provided which can be configured to receive at least one signal that is associated with the emitted electromagnetic radiation, and reflect and/or deflect the signal(s) to a further location. In addition, a laser resonating system can be provided which can form an optical circuit, and be configured to control a spatial mode of the emitted electromagnetic radiation. The exemplary apparatus can cause the emitted electromagnetic radiation to propagate substantially unidirectionally within at least one portion of the laser resonating system.

Another exemplary embodiment of an apparatus according to the present disclosure can be provided. In this exemplary embodiment, at least one arrangement can also be provided which is configured to emit an electromagnetic radiation. Such exemplary radiation can have (i) a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond, and (ii) an instantaneous line width that is less than about 15 gigahertz.

According to still exemplary embodiment, at least one arrangement can be provided configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, with the periodic selection being performed at a first characteristic period. The mean frequency can vary linearly over time, wherein the apparatus can emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period. Further, the first characteristic period can be greater than the second characteristic period.

For example, the first characteristic period can be at least two times that of the second characteristic period. The second electromagnetic radiation(s) can have a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond. The second electromagnetic radiation(s) can also have an instantaneous line width that is less than about 15 gigahertz.

According to still another exemplary embodiment of the present disclosure, an apparatus can be provided. The exemplary apparatus can include at least one arrangement which can be configured to select at least one first electro-magnetic radiation based on a mean frequency of the first electro-magnetic radiation(s). The selection can be performed by the arrangement(s) with a first characteristic free spectral range. For example, the apparatus can emit at least one second electromagnetic radiation that can have a spectrum whose mean frequency changes periodically as a function of time with a second characteristic free spectral range. The first characteristic free spectral range can be greater than the second characteristic free spectral range.

According to one exemplary variant of the present disclosure, the first characteristic period can be at least two times that of the second characteristic period. The second electromagnetic radiation(s) can have a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond. The second electromagnetic radiation(s) can also have an instantaneous line width that is less than about 15 gigahertz. The mean frequency can vary linearly over time.

In still a further exemplary embodiment, an apparatus can be provided. This exemplary apparatus can include at least one arrangement which is configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the first electro-magnetic radiation(s). The periodic selection can be performed at a first characteristic period. The exemplary apparatus can also be configured to emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period. For example, the first characteristic period can be greater than two times the duration of the second characteristic period.

These and other objects, features and advantages of the exemplary embodiment of the present disclosure will become apparent upon reading the following detailed description of the exemplary embodiments of the present disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the present disclosure, in which:

FIG. 1 is a schematic diagram of a first exemplary embodiment of a high-speed wavelength-swept laser system/arrangement with a unidirectional ring resonator according to the present disclosure;

FIG. 2 is a combinational block and functional diagram illustrating an exemplary use of a time interleaving optical delay line and a booster amplifier, according to an exemplary embodiment of the present disclosure;

FIG. 3( a) is a diagram of an exemplary embodiment of a time interleaving fiber delay line with a cascaded Mach-Zehnder interferometer for copying and pasting of the laser output, according to the present disclosure;

FIG. 3( b) is a diagram of another exemplary embodiment of the time interleaving fiber delay line with a tree-like fiber delays with Faraday rotator mirror for copying and pasting the laser output, according to the present disclosure;

FIG. 4 is a schematic diagram of a second exemplary embodiment of the high-speed wavelength-swept laser system/arrangement with a unidirectional ring resonator, according to the present disclosure;

FIG. 5 is a schematic diagram of an third embodiment of the high-speed wavelength-swept laser system/arrangement with a linear cavity resonator, according to the present disclosure; and

FIG. 6 is a schematic diagram of a fourth exemplary embodiment of the high-speed wavelength-swept laser system/arrangement with the linear cavity resonator, according to the present disclosure.

Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject disclosure will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described exemplary embodiments without departing from the true scope and spirit of the subject disclosure as defined by the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic diagram of an exemplary embodiment of a high-speed wavelength-swept laser system/arrangement in accordance with the present disclosure. For example, the exemplary laser system/arrangement can comprise an optical high-finesse wavelength tuning filter 1′ that can utilize a polygon scanning mirror 700, a short length unidirectional ring resonator cavity 2′, and a time interleaving optical delay line 400 with a booster optical amplifier 500. The exemplary optical wavelength tuning filter 1′ can be configured as a reflection-type filter which can have substantially similar or identical input and output ports. The exemplary wavelength tuning filter 1′ can include a diffraction grating 220, a telescope with a set of lenses 260 and 262, a polygon scanning mirror 700, and end reflectors 280 and 282.

The exemplary implementation of the wavelength tuning procedure according to an exemplary embodiment of the present disclosure can be similar to the previous approach [see description of Oh, Optics Letters 30(23), 3159-3161 (2005) and disclosure of International Patent Publication WO 2005/001401] including the exemplary procedure of four times reflection on the polygon mirror (e.g., twice×double pass) for large finesse of the filter but without a folded telescope according to the exemplary embodiment of the present disclosure to reduce the optical path length. In the conventional approach, a polygon scanning mirror with a large number of facets can be used to increase the tuning repetition rate. Although increasing the number of facet may assist with increasing the tuning repetition rate, it may not be the most efficient way to increase the wavelength sweep rate with given filter bandwidth, which can be characterized by the total wavelength sweep range per unit time and unit width of the filter pass-band.

For example, with fixed filter bandwidth, the finesse of a polygon scanning filter can be inversely proportional to N²,

${F = {\frac{\left( {F\; S\; R} \right)}{\left( {\delta\;\lambda} \right)_{3\; d\; B}} = {\frac{\pi^{3}}{\sqrt{\ln\; 2}}\frac{D}{\lambda\; N^{2}}}}},$ where (FSR) is the free spectral range and (δλ)_(3dB) is the 3 dB bandwidth of the filter, respectively, D is the diameter of the polygon, and N is the number of facet of the polygon. Therefore, the wavelength sweep rate can generally become proportional to 1/N. With the conventional polygon based wavelength-swept laser with N=128 providing 115 kHz tuning repetition rate (9,200 nm/ms), it is possible to obtain about 41,800 nm/ms wavelength sweep rate, e.g., if a polygon mirror 700 is used with N=28. According to the exemplary embodiment of the present disclosure, the tuning repetition rate can be about 25 kHz can be used. However, the FSR (which determines the wavelength tuning range) can become about 1,664 nm.

When the gain in the laser cavity supports lasing over ˜104 nm bandwidth, a continuous wavelength sweep over about 104 nm can be obtained with about 6.25% duty cycle 322. An idle portion of the exemplary laser output can then be completely filled up by making 15 copies of the laser output, properly delaying, and pasting with the time interleaving optical delay line 400, resulting in, e.g., about 16 wavelength sweeps in a single facet-to-facet rotation period of the polygon 700. As a result, the repetition rate of about 400 kHz (25 kHz×16) can be obtained. The output of the delay line 400 can be amplified using the booster-amplifier 500.

When wavelength sweep rate is high, a reduction of an optical path length of the laser cavity can become important. For example, in order to support long photon intracavity lifetime by reducing the filter center wavelength shift per cavity round trip, the length of the laser resonator can be reduced and/or minimized by using a short focal length lenses for the telescope (e.g., lenses) 260, 262. In the ring cavity 2′, an isolator, which facilitates a unidirectional lasing, can be integrated inside the semiconductor optical amplifier (SOA) gain medium 100, and fiber pigtails 120, 122 of the SOA 100 can be re-connetorized at ends/connectors 140, 142, thus leaving a minimum length of the optical fiber. In order to form a ring oscillation, the output of the polygon filter can be vertically offset and directed to the input connector 142 of the SOA 100 by the reflector 200. Additional components of the exemplary embodiment shown in FIG. 1 and the exemplary operation thereof shall be described herein below.

FIG. 2 shows a combinational block and functional diagram of an exemplary embodiment according to the present disclosure which illustrates an exemplary use of a time interleaving optical delay line and a booster amplifier. For example, the laser output 246 can be provided directly from the polygon filter 820, for example, that can have about 25% duty cycle in this exemplary case. Three copies of this output can be directed and made at 244, properly delayed, and then pasted 840 through the time interleaving delay line 400. Laser output power 600 can then be recovered 860 by a booster amplification 500.

FIGS. 3( a) and 3(b) show exemplary embodiments of the time interleaving optical delay line with optical fibers for copying and pasting of the laser output according to the present disclosure. To make three copies, for example, three different lengths of fiber delays, 442, 444, and 446 can be utilized. These exemplary delayed copies can be pasted to the original laser output 246 by using, e.g., a cascaded Mach-Zehnder interferometer as shown in the exemplary embodiment of FIG. 3( a) (which also illustrates the use of splitter 420, 422, 424 and the output provided on line 402). It is also possible to utilize a tree-like interferometer with a circulator 403, splitter 421, 423, 425 and Faraday rotator mirrors 460, 462, 464, and 466, so as to reduce or eliminate a birefringence effect in the delay line, as shown in the exemplary embodiment of FIG. 3( b).

FIG. 4 shows a schematic diagram of a second exemplary embodiment of the high-speed wavelength-swept laser system/arrangement according to the present disclosure. Using this exemplary embodiment, the output of the laser cavity can be obtained after the SOA (e.g., gain medium) 100, and through the free-space output beam splitter 240. Since a filtered light 320 can be amplified by the SOA 100 before being coupled at point 300, the output light from the cavity 302 can contain, e.g., a particular amount of a amplitude spontaneous emission (ASE) during an idle portion of the output. To prevent or reduce such ASE from being overlapped with the copied laser output in the delay line/arrangement 400, the SOA gain medium 100 can be modulated to be switched off at point 102 during the idle period of the laser resonator output.

FIG. 5 shows a schematic diagram of a third exemplary embodiment of the high-speed wavelength-swept laser system/arrangement with an exemplary linear cavity configuration according to the present disclosure. For example, a SOA gain medium 110 having a mirror coating 122 can be provided on one end of the system/arrangement which can support a linear cavity oscillation of the laser arrangement. The output from the oscillator of the laser can be coupled by the Free-space optic beam splitter 240.

FIG. 6 shows a schematic diagram of a fourth exemplary embodiment of the high-speed wavelength-swept laser system/arrangement having linear cavity configuration according to the present disclosure. In this exemplary embodiment, the SOA gain medium 112 having a partial reflection coating 124 can be provided on one end of the exemplary system/arrangement which can support a linear cavity oscillation of the laser and the transmitted light through the partial reflector 124. Such SOA gain medium 112 can be coupled as a laser output, and the directed light can be directed 244 to the optical delay line 400. The ASE in the idle portion of the laser resonator output can be reduced and/or eliminated by the gain switching 102 of the SOA 112.

The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Indeed, the arrangements, systems and methods according to the exemplary embodiments of the present disclosure can be used with and/or implement any OCT system, OFDI system, SD-OCT system or other imaging systems, and for example with those described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004 which published as International Patent Publication No. WO 2005/047813 on May 26, 2005, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005 which published as U.S. Patent Publication No. 2006/0093276 on May 4, 2006, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004 which published as U.S. Patent Publication No. 2005/0018201 on Jan. 27, 2005, and U.S. Patent Publication No. 2002/0122246, published on May 9, 2002, the disclosures of which are incorporated by reference herein in their entireties. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present disclosure. In addition, to the extent that the prior art knowledge has not been explicitly incorporated by reference herein above, it is explicitly being incorporated herein in its entirety. All publications referenced herein above are incorporated herein by reference in their entireties. 

What is claimed is:
 1. An apparatus comprising: at least one wavelength swept laser arrangement configured to emit an electromagnetic radiation that has a spectrum whose mean frequency changes (i) at an absolute rate that is greater than about 6000 terahertz per millisecond, and (ii) over a range that is greater than about 10 terahertz, wherein the spectrum has an instantaneous line width that is smaller than 100 gigahertz.
 2. The apparatus according to claim 1, wherein the mean frequency changes repeatedly at a repetition rate that is greater than 5 kilohertz.
 3. The apparatus according to claim 2, wherein the spectrum has a tuning range whose center is approximately centered at 1300 nm.
 4. The apparatus according to claim 2, wherein the spectrum has an instantaneous line width that is smaller than 35 gigahertz.
 5. The apparatus according to claim 1, further comprising a laser resonating system forming an optical circuit, and configured to control a spatial mode of the emitted electromagnetic radiation.
 6. The apparatus according to claim 5, wherein the apparatus causes the emitted electromagnetic radiation to propagate substantially unidirectionally within at least one portion of the laser resonating system.
 7. An apparatus, comprising: at least one wavelength swept laser arrangement configured to emit an electromagnetic radiation that has a spectrum whose mean frequency changes (i) at an absolute rate that is greater than about 6000 terahertz per millisecond, and (ii) over a range that is greater than about 10 terahertz; and a polygon arrangement which is configured to receive at least one signal that is associated with the emitted electromagnetic radiation, and at least one of reflect or deflect the at least one signal to a further location.
 8. An apparatus comprising: a wavelength swept laser arrangement configured to emit an electromagnetic radiation that has (i) a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond, and (ii) an instantaneous line width that is less than about 15 gigahertz; and a laser resonating system forming an optical circuit, and configured to control a spatial mode of the emitted electromagnetic radiation.
 9. The apparatus according to claim 8, further comprising a polygon arrangement which is configured to receive at least one signal that is associated with the emitted electromagnetic radiation, and at least one of reflect or deflect the at least one signal to a further location.
 10. The apparatus according to claim 8, wherein the apparatus causes the emitted electromagnetic radiation to propagate substantially unidirectionally within at least one portion of the laser resonating system.
 11. An apparatus comprising: at least one wavelength tuning filter arrangement configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, the periodic selection being performed at a first characteristic period, wherein the mean frequency varies linearly over time, wherein the apparatus is configured to emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period, and wherein the first characteristic period is greater than the second characteristic period.
 12. The apparatus according to claim 11, wherein the first characteristic period is at least two times that of the second characteristic period.
 13. The apparatus according to claim 11, wherein the at least one second electromagnetic radiation has a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond.
 14. The apparatus according to claim 13, wherein the at least one second electromagnetic radiation has an instantaneous line width that is less than about 15 gigahertz.
 15. The apparatus according to claim 11, wherein the at least one arrangement includes a polygon scanning arrangement.
 16. An apparatus comprising: at least one wavelength tuning filter arrangement configured to select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, the selection being performed by the at least one arrangement with a first characteristic free spectral range, wherein the apparatus emits at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic free spectral range, and wherein the first characteristic free spectral range is greater than the second characteristic free spectral range.
 17. The apparatus according to claim 16, wherein the first characteristic period is at least two times that of the second characteristic period.
 18. The apparatus according to claim 16, wherein the at least one second electromagnetic radiation has a spectrum whose mean frequency changes at an absolute rate that is greater than about 2000 terahertz per millisecond.
 19. The apparatus according to claim 18, wherein the at least one second electromagnetic radiation has an instantaneous line width that is less than about 15 gigahertz.
 20. The apparatus according to claim 16, wherein the mean frequency varies linearly over time.
 21. The apparatus according to claim 16, wherein the at least one arrangement includes a polygon scanning arrangement.
 22. An apparatus comprising: at least one wavelength tuning filter arrangement configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, the periodic selection being performed at a first characteristic period, wherein the apparatus is configured to emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period, and wherein the first characteristic period is greater than two times the duration of the second characteristic period.
 23. The apparatus according to claim 22, wherein the at least one arrangement includes a polygon scanning arrangement. 